EP 1451991 describes MIMO (Multiple Input Multiple Output) transmission and reception. MIMO transmission involves transmission of a data stream distributed over a plurality of parallel transmitters. This serves to increase transmission capacity compared to transmission from a single transmitter. The transmissions of the transmitters are received by a plurality of receivers and the received signals are used to reconstruct the data stream.
In a MIMO system, reconstruction involves correction for interference between transmissions from different transmitters. Reconstruction is made possible by encoding the data stream before transmission, using an error correcting code (ECC) and interleaving. The ECC adds redundancy that ensures that errors can't change a code symbol for one data input into a valid code symbol for another data input, unless more than a threshold number of errors occurs in the code symbol. Interleaving ensures that different bits from the same code symbol are not located close to each other, so that it is avoided that they can be collectively affected by a short burst of errors.
After reception, the information from the receivers is de-interleaved and used to determine the most likely code words in view of the received information from the combination of receivers. EP 1451991 describes a MIMO system wherein an iterative process is used to determine the most likely code words. This involves determining the likelihood of possible transmitted code bits from the received information and a priori probabilities of the code bits. The a priori probabilities are determined iteratively by a bit-based ECC decoder.
EP 1451991 uses an iterative loop for this purpose. At the input of the loop the receivers supply numbers, each for a respective one of the bits, to represent the likelihood that the bit is set. Numbers representing corrected likelihoods for the bits are obtained by subtracting a priori likelihoods for the bits, obtained from feedback in the iterative loop. A decoder uses the combination of likelihoods for the bits of code symbols to produce the a priori likelihoods for use in the feedback. A de-interleaver is used in front of the decoder to reorder the corrected numbers in order to re-assemble the likelihoods for the different bits of a code symbol. Behind the decoder an interleaver is used, to interleave the priori likelihoods from the decoder back to the order of the bits at the input of the iterative loop.
This approach does not make optimal use of all information about reception conditions. The de-interleaving separates information about different bits that are received in the same time slot, before supplying the information to the decoder. As a result the bit-based ECC decoder generates bit information with optimally improved reliability only if the bits and their respective information are independent among each other. However, this assumption of independency is not always true. An example of situations where dependency among bits in the same codeword exists is slow fading. Another example is in a MIMO system where the signals transmitted by different transmission antennas interfere with each other, due to the cross-correlation among MIMO channels. In such situations, information about correlations between such bits, e.g. the joint probabilities of these bits and/or between the errors that occur in such bits may be exploited to reduce the symbol error rate. However, it is difficult for the prior art bit-based decoder to use such correlation information.
Many prior art documents describe the use of bit level interleaving between detection of received data in terms of probabilities and decoding by means of an error correcting code. This means that relations between bits received in the same time slot are obscured for decoding according to the error correcting code. This is the case for example in WO02/062002, US2008/058109 and WO03/049397, which all describe MIMO systems that use an error correcting code and bit level interleaving before error correction using the error correcting code.
U.S. Pat. No. 7,245,666 concerns MIMO coding using soft symbol decoding phase, wherein received data is compared to possible combinations of transmitted symbols, to generate a soft probability metric for each of the transmitted bits. The space of possible combinations of transmitted symbols with which the received data is compared is reduced by limiting the number of bit combinations for individual antennas. The soft metrics for the transmitted bits is used as input for a Viterbi decoder that performs hard symbol decoding.
Although U.S. Pat. No. 7,245,666 uses soft metrics for individual bits as input for hard symbol decoding, it first computes joint metrics for multi-bit symbols. These are reduced to a metric for an individual bit at an individual antenna, by taking the minimum value of the joint metric over bit combinations wherein the bit has the same value. The metrics for the individual bits are supplied to the Viterbi decoder. Thus, information about joint detection probability is suppressed before hard decoding.
WO2004/071001) also concerns MIMO decoding of received symbols. An example is given how the demodulation output is converted into probability values for individual bit symbols, eliminating joint probabilities. Joint probability effects such as inter symbol interference are eliminated at an earlier stage, by means of a space matched filter. A Reed Solomon decoder may be used, but the code symbols of the Reed Solomon code appear to be unrelated to individual time slots.